About 40 years ago, when polymer chemists, physicists and engineers began to realize there was a need for prosthetic devices that can be implanted safely into the blood stream, the blood vessel wall appeared to use only very few simple properties for maintaining the fluidity of blood in vivo. Simple probes for these properties yielded correspondingly simple answers, so that a host of materials was created to imitate the simple vascular properties found. Then, biologists and hematologists noticed that in vitro, blood in contact with these materials was provoked to react in a variety of ways. Visible reactions: fibrin formation, platelet adhesion, platelet spreading and aggregation, adhesion and spreading of specific white blood cells each could occur, and each now appears to result from specific, rapid plasma protein interactions at the interface. Nine years ago, 2 of the present 3 sponsors organized a conference similar to the one now proposed. The resulting publication (Ann N.Y. Acad. Sci. 283, pp 1-560. 1977) contains theories, e.g. concerning protein conformation, that since led to hard data; and it contains data, e.g. concerning substances affecting platelets, that are now applied clinically; but it also contains erroneous statements. The realization is beginning to evolve, that implantation of a device such as an artificial heart, initiates a vast network of events that may lead from minimal endothelial damage or protein adsorption and interactions within seconds, to cellular and tissue changes, immune reactions, thrombosis, embolus formation and death in days or months. Thus, while the 1977 Annal is widely used, it is outdated. New tools are probing deeper into a wider range of special fields, and renewed efforts are required to coordinate the new data and theories ranging from the work of pure physicists to that of practicing surgeons. The new Annal that will result from the meeting proposed here should provide a better guide in the continued search for blood compatible biomaterials.